1. Field of Invention
This invention relates to a solvent-free, low monomer or monomer-free, polymerisable and radiation-hardenable melt composition for coating members of the group consisting of planar substrates, formed bodies of metal, plastic, cellulose material, inorganic material and a method for its production. The melt composition according to the invention is particularly suited to the corrosion-and-abrasion resistant finishing of planar packaging material substrates, preferably those used in the foodstuffs and pharmaceutical industries. Substrates finished with the melt composition according to the invention can be utilized in other areas, e.g. in the manufacture of automobile bodies, construction and in similar industrial applications.
2. Description of the Related Art
Corrosion-and-abrasion resistant finishing or treatment of planar substrates of metal, plastic and cellulose materials is a technical requirement that is defined by the materials to be treated. The purpose of the treatment is to protect objects against environmental damage and thus extend their lifetime. Protection against corrosion now plays an important role in the economy. In packaging materials, particularly those used in the foodstuff and pharmaceutical industries, additional high requirements are demanded of the anti-corrosion composition. The latter must be physically and chemically inert in the presence of a packaged product and must not release any toxic substance(s), or any substance(s) capable of altering taste or odour characteristic, into the surrounding area. In order to meet these requirements, state-of-the-art finishing systems must incorporate solvent-containing coating materials and lacquers. In this product group, the solvents serve as important auxiliary substances in that they enable the anti-corrosion substances consisting of polymers and resins to be utilized since the raw or starting materials for the latter are available primarily only as solids. The fluid state of the coating substance is further useful in that it "wets" the surface to be protected which is an important precondition for adhesion.
The solvents required for the above process may be aliphatic or aromatic solvents, such as e.g. esters, ketones, toluols, xylols, and the like substances. After the coating is applied and allowed to set, the solvents must be removed from or driven out of the protective coating material. Years ago, these solvents were simply driven off and permitted to escape into the atmosphere. However, legislation in the past few years has restricted such emissions. As a consequence, recycling processes have been proposed to attempt to recover the solvents. Despite the relatively high yield afforded by such recovery plants and the impressive practical rate of recovery (up to 95%), such processes are highly capital intensive in terms of investment and operation. Additionally, they also entail the disadvantage that the solvent-containing emissions, in order to conform to statutory air purity requirements, must be treated by after-burning. A further problem related to solvent recycling is that many of these solvents consist of mixtures of two or more solvents and therefore cannot be readily used with other coating materials or lacquers. Also, the recovered solvents and their mixtures are not capable of unlimited application, which is to say that after some recycling processes, such solvents must be eliminated, e.g. burnt off. In summary, these treatment processes are technically and economically quite burdensome and practically in no case are they 100% effective.
A partial improvement in the coating or treatment process has been achieved by using anti-corrosion agents having an aqueous dispersion base with a high solids content. With regard to the aqueous anti-corrosion agents, however, it has been demonstrated in many industrial sectors that the use of organic solvents cannot be completely dispensed with. Thus, the problem of solvent recovery originated. This affects primarily electrical coating by dipping where dispersion lacquers with solvent contents of up to 20% relative to the finished product are employed.
Recently, however, solvent-free coating materials and lacquers have also become known These substances employ the so-called "one-pot" and "multi-pot" systems. In the one-pot system, ready for application, the material becomes re-activated and cross-linked in the presence of air moisture, while in the case of a two or more pot system, the product can be used only after homogeneous mixing. Due to the fact that these anti-corrosion materials ready for application possess a so-called "pot-time" in which they can be applied, such materials have, for industrial application purposes, a series of disadvantages which cannot always be compensated for by the use of mixing and dispensing apparatus. Since they must furthermore be present as solvent-free, anti-corrosion materials in a fluid aggregate state, their individual reactive components consist of relatively low molecular compounds. These low molecular weight compounds, known also as reactive diluents, are in many ways not only physiologically harmful but possess distinct and characteristic negative taste and smell characteristics. The degree of cross-linking achievable, being only in the vicinity of .ltoreq.90% ensures amongst other matters that these products cannot be used to finish packaging materials for foodstuffs or pharmaceuticals. Technically speaking, however, too low a degree of cross-linking may hamper resistance against environmental influences. In general, the reactive diluents have the disadvantage that, when residues, even in parts per million amounts, are not cross-linked, they negatively influence adhesion to the boundary surface, since they are able to disperse much like solvents.
In order to accelerate the curing or the cross-linking process, polymerisable coating materials and lacquers have become known, which can be hardened by ionising radiation, notably electron and UV radiation.
In order for coating materials and lacquers to harden in the presence of UV-radiation, so-called photoinitiators and even synergists must be added. These photosensitive additives, however, after curing remain in the anti-corrosion layer and, during stacking operations or if in contact with packed product, contaminate the latter and as they are considered to be harmful to health, cannot be used for food or pharmaceutical packaging. The curing procedure involving ionizing radiation, however, does not require the addition of photosensitive substances or synergists. The state-of-the-art radiation-hardenable coating materials and lacquers present another considerable disadvantage in that they must, for processing purposes, have a relatively high acrylic group monomer content, the latter serving as application-facilitating reactive diluents. While some acrylic monomers are potentially harmful, others are known to be toxic. The existence of taste-and-smell-influencing substances that either escape into the atmosphere or come into contact with the packaged product, particularly foodstuffs and pharmaceuticals, remains one of the greatest disadvantages; it can be caused by the presence of very small amounts of monomer residue, even in the very low parts-per-million range.
Such anti-corrosion materials provide no certainty to technologists seeking in the future to finish objects under environmentally compatible and hygienic conditions, since the present state of the art provides no solutions that can meet these requirements. High standards have already been set regarding food and pharmaceuticals packaging, examples of which can be found in directives issued by the Federal Health Department (Germany) in its publication "Kunststoffe im Lebensmittelverkehr" ("Plastics in the Food Handling Industry"), and in directives published by the Food and Drug Administration and in various other national environmental codes. A further problem is caused by the additives required to produce conventional anti-corrosion materials. This has been described in Gachter/Muller "Kunststoff-Additive" 2. Ausgabe, Hanser-Verlag, Munchen, 1983, im Kapitel 18 "Gewerbe-und lebensmittel-hygienische Aspekte von Kunstoff-Additiven" ("Plastic Additives" 2nd Edition, Hanser-Verlag, Munchen, 1983, volume 18 of "Hygienic Aspects of Plastic Additives in Industrial and Foodstuffs Sectors"). This subject has been more fully explored in the paper by Piringer et al. on "Der Einfluss von Restlosehmitteln und monomeren Acrylaten aus Verpackungen auf die sensorischen Eigenschaften von Lebensmitteln" ("The Effect of Residual Solvents and Monomer Acrylates in Packaging Materials on the Sensory Properties of Foodstuffs"), in Verpackungsrundschau, Issue 8/1986, pp 53-58, since the premise had already been established that the residual solvents and the acrylic monomers have a particular sensory influence on the packaged food products. This paper demonstrates that, when inert solvents, acrylates or methacrylates are employed, the indicated relative threshold values of the latter for smell and taste remain problematic when such low-molecular compounds are used. Thus, the relative threshold value, for example in the case of n-butylacrylate is 0.002 and in the case of 2-ethyl-hexylmethacrylate is 0.02 mg/kg.
It can furthermore be said that, aside from differences in approach, the evaluation of the statutory measures and regulations concerning the environment, and industrial and personal hygiene, are substantially identical in the European Common Market, the American Market and in Japan. Comprehensive and comparative explorations of this area have been published by Keener, R. L., Plamondon, J. E. and West, A. S. in "Recent Developments in the Regulation of Industrial Chemicals in the United States and Europe", presentation by RADCURE EUROPE '85, Basel/Switzerland, Sponsor: AFP/SME, Dearborn, Mich. 48121, U.S.A. and in the book by Ronald Brickman et al. "Controlling Chemicals: The Politics of Regulation in Europe and the United States" Cornell University Press, Ithaca, N.Y., 1985.
Conventional lacquers and varnishes with different polymer bases which are dissolved in solvents have been amply described in the literature, as, for example in H. Kittel "Lehrbuch der Lacke und Beschichtungen" Bd. 4, 5 und 7, (Introduction to Lacquers and Coatings Vols. 4, 5 and 7 ) Verlag W.A. Colomb Verlagsgesellschaft mbH, Berlin und Oberschwandorf. Reference may be made to this publication for that purpose, and accordingly, it will not be necessary to consider this prior art in particular.
Similarly, corrosion-and-abrasion-resistant finishing of planar substrates and/or formed bodies (especially those containing metal and cellulose materials) which comprise coating agents without inert solvents, is well known and understood in industrial practice. In this connection, coating substances were employed, whereby either the backbone polymers were dissolved in reactive diluents or the base products were sufficiently fluid to be applied. Although these reactive diluents and/or other fluid co-reactants are integrated in the polymer matrix either by hardening or by cross-linking, residues remain, the amount of which depends upon the degree of cross-linking attained These non-integrated residues can neither be removed by additional costly cleansing processes, nor can they be reduced to amounts permissible under existing regulations. Because such compositions are also potentially harmful to the human body, such anti-corrosion materials have only a limited applicability, the implication of which being that such materials must be entirely excluded from use in the pharmaceutical and foodstuffs industries.
This is due solely to the fact that the sensory qualities (taste and smell) of such packaged products are easily affected. Although qualitative improvements may be made to non-thermosensitive substrates by baking and/or subsequent curing, such measures are seldom capable of ensuring compliance with required minimum standards. To this must be added the cost of such after-treatment, which is in turn reflected in the cost of the final product. Efforts have been made, therefore, to discover improved and economically sounder alternatives to radiation hardening, which would at the same time improve the product marketability. Due to the costs of attempts to meet minimum cross-linking levels--as has already been discussed, the radiation-hardenable coatings compositions have not produced the desired breakthroughs. Thus, in European patent application 0 157 396 thus are described radiation-hardenable compositions for sheet steels, which remain moldable after curing. Due to the fact, however, that these materials are processed at room temperature, they possess a relatively high proportion of reactive diluents, i.e. low-molecular acrylic monomers. Apart from featuring a relatively high residual monomer content, these radiation-hardened coating materials have only a limited workability or moldability. Experts are aware that, as the proportion of short-chain, low-molecular monomers in a coating matrix increases, so does toughness and brittleness. During processing, a great number of hairline cracks can appear in the anti-corrosion film, which significantly reduces product effectiveness.
In European Patent Application 0 184 349, radiation-hardenable, thermoplastic coating materials for wood and other substrates are described, which consist of copolymerisable ethylenically unsaturated polyesters and thermoplastic polymers. In order to be able to process these as coating materials, monomers, i.e. reactive diluents and/or inert organic solvents, are required. In this regard, coating materials have been proposed which, while offering good end product qualities, do not overcome the problems of inert solvent evaporation nor solvent and monomer residues.
For the purpose of anti-corrosion finishing, so-called "hot melt" compositions are known, whose structure is based upon inert resins, waxes, thermoplastics and/or elastomers. Use of the term "hot" is considered inappropriate (see Rompp's Chemie-Lexikon, 8. Aufl., Bd. 3/1983, s. 1763 (Rompp's Chemical Dictionary, 8th Edition., Volume 3/1983, page 1763), and accordingly, reference only to melt compositions will be used herein.
Melt adhesives, which are related to melt compositions, have achieved prominence in many sectors, but melt compositions have remained relatively unknown apart from their application in some areas such as anti-corrosion films. The latter are produced by using a dip of the melt composition, which may include cellulose esters, plasticizing mixtures and mineral oil additives, so that e.g. equipment or machine parts, are dipped in the hot composition and then left to cool. The film or coating thus developed may then be removed without leaving any residue.
Whether the melt compositions are employed for coating or for adhesive purposes, the thermoplastic raw materials, which include resins and plasticizers, are thermosensitive and hence subject to thermal oxidation, particularly in the presence of atmospheric oxygen. In this case, not only are the properties of the product altered, but physiologically harmful crack-products are also produced. The thermal problems connected herewith are described by the internationally used term "Heat History". Whereas with melt adhesives stabilizers and antioxidants can be used, the latter can be used with melt compositions only after they have been accepted for use in the technical sense. Such thermo-oxidative decomposition can be minimized by masking with inert gases such as nitrogen (N.sub.2). A further disadvantage of using thermoplastic melt coating materials is that of their relatively low plasticization points, which should lie below +150.degree. C., particularly below +120.degree. C. A further disadvantage in this regard is that the backbone polymers in their end state are already macromolecules and therefore require very high process temperatures, from 180.degree. C. to +270.degree. C., in order to achieve sufficient wetting and thus adhesion to the various substrate surfaces. While materials exist that remain molten at lower temperatures, such materials possess no thermal stability and little chemical resistance. Such melt coating materials, which are for example described in DE-OS 24 25 395, have a formula based on ethylene-vinylacetate copolymers. Other melt compositions are described in the monograph by R. Jordan "Schmelzklebstoffe", Bd. 4a/1985 und Bd. 4b/1986, ("Melt adhesives"), Vol. 4a/1985 and Vol. 4b/1986, HINTERWALDNER VERLAG, Munchen". Included in the discussion are polyester melt compositions whose structures, based on linear copolyesters of terephthalic and/or isophthalic acid, can range from amorphous to crystalline. (DE-OS 24 14 287).
In order to better control the critical parameters which influence the "Heat History" and at the same time to improve the end properties such as thermal stability, reactive melt compositions have been proposed. Such melt compositions were mainly adhesive and sealing materials, which although processed analogously to melt coating material from a molten mass at temperatures below .ltoreq.150.degree. C., especially .ltoreq.100.degree. C. and therefore workable at an early stage, underwent cross-linking only in the presence of atmospheric humidity. Such materials are ideally moisture-hardened polyurethane systems. Depending on the layer thickness and the surrounding humidity, the curing process requires from 1 to 96 hours for completion. Such a curing process is, from the industrial standpoint, an immutable necessity even though un-integrated curing components are able to migrate To date, other reactive melt compositions have neither been treated in the literature nor known in industry, wherefore the reasons cannot be solely those related to the so-called "Heat History", but are to be found in the unavailability suitable raw materials. Additional preventive measures alone are not sufficient to ensure compliance with increasingly demanding standards for environmental protection, food processing hygiene, workplace hygiene and sensory effects on packaging materials, since such measures entail both higher investments in plant, measuring equipment, etc. as well as expensive control systems.
For improved environmental protection and better workplace and food handling hygiene, it would be advantageous to address existing and potential product deficiencies by eliminating as far as possible their causes and thus decrease costs. Despite the numerous attempts already made, it has not been possible to manufacture melt compositions that possess even a fraction of the properties attributed to materials containing organic solutions. Since, however, corrosion-resistant finishes have, for economic reasons, already become a matter of public concern and necessity, the present invention is aimed at identifying innovative processes for the future which will eliminate and/or reduce the above-discussed and other deficiencies, while rectifying and/or reducing sensory problems and those related to food packaging and workplace hygiene.